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Abstract:

The disclosure is directed toward driving methods and a driving circuit
which are particularly suitable for bi-stable displays. In certain
embodiments, methods provide the fastest and most pleasing appearance to
the desired image while maintaining the optimal image quality over the
life expectancy of an electrophoretic display device.

Claims:

1. A method for driving a display device comprising a plurality of
pixels, the method comprising: applying a sequence of voltage potential
across a display medium for each pixel of the plurality of pixels such
that time integrals of net magnitudes of the sequences of voltage
potentials are substantially equal for all pixels of the plurality of
pixels over a predetermined time period.

2. The method of claim 1, wherein the display medium is an
electrophoretic fluid.

3. The method of claim 1, further comprising: applying a correction
waveform if time integrals of net magnitudes of the sequences of voltage
potentials are not substantially equal for all pixels of the plurality of
pixels over a predetermined time period.

4. A method for driving a display device comprising a plurality of
pixels, the method comprising: applying a sequence of driving pulses to
each pixel of the plurality of pixels to cause resets of the pixel.

5. The method of claim 4, wherein a total number of resets to a first
color state and a total number of resets to a second color state are
substantially equal, for the pixel over a predetermined time period.

6. The method of claim 4, wherein sums of resets regardless of color
states are substantially equal, for all pixels of the plurality of
pixels.

7. The method of claim 4, further comprising: resetting the pixel to a
predetermined color state after a predetermined number of driving pulses
have been applied to the pixel.

8. The method of claim 7, wherein all pixels of the plurality of pixels
are reset to the predetermined color state at about the same time.

9. The method of claim 7, wherein the predetermined numbers of driving
pulses are substantially equal for all pixels of the plurality of pixels.

10. The method of claim 4, further comprising: resetting the pixel to a
predetermined color state after the sequence of driving pulses have been
applied to the pixel for a predetermined amount of operating time.

11. The method of claim 4, wherein total numbers of resets to a specific
color state among a plurality of color states are substantially equal,
for all pixels of the plurality of pixels.

12. The method of claim 4, further comprising: applying a correction
waveform to correct an imbalance.

13. A method for driving a display device comprising a plurality of
pixels, the method comprising: applying a correction waveform to correct
a DC imbalance.

14. The method of claim 13, wherein the correction waveform is applied
when the display device is not in use.

15. The method of claim 13, wherein the correction waveform is applied to
correct a positive or negative DC imbalance.

16. The method of claim 13, wherein the correction waveform is applied
between a clearing phase and a driving phase of the display device.

17. The method of claim 13, wherein the correction waveform is applied
after a driving phase of the display device.

18. The method of claim 13, wherein the correction waveform is applied
without interfering with driving of the plurality of pixels for
displaying intended images.

19. The method of claim 13, wherein the applying comprises: applying a
correction waveform to correct a imbalance so that a time integral of an
average voltage potential applied across the display device is
substantially zero over a period of time.

Description:

REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation of U.S. patent application Ser. No.
12/132,238 filed on Jun. 3, 2008, which claims the benefit under 35 USC
§119(e) of provisional application 60/942,585, filed Jun. 7, 2007,
the entire contents of which are hereby incorporated by reference for all
purposes as if fully set forth herein.

TECHNICAL FIELD

[0002] The present disclosure relates to an electrophoretic display, and
more specifically, to driving approaches and circuits for an
electrophoretic display.

BACKGROUND

[0003] An electrophoretic display (EPD) is a non-emissive bi-stable output
device which utilizes the electrophoresis phenomenon of charged pigment
particles suspended in a dielectric fluid to display graphics and/or
alphanumeric characters. The display usually comprises two plates with
electrodes placed opposing each other. One of the electrodes is usually
transparent. The dielectric fluid which includes a suspension of
electrically charged pigment particles is enclosed between the two
plates. When a voltage potential is applied to the two electrodes, the
pigment particles migrate toward the electrode having an opposite charge
from the pigment particles, which allows viewing of either the color of
the pigment particles or the color of the dielectric fluid.
Alternatively, if the electrodes are applied the same polarity, the
pigment particles may then migrate to the one having a higher or lower
voltage potential, depending on the charge polarity of the pigment
particles. Further alternatively, the dielectric fluid may have a clear
fluid and two types of colored particles which migrate to opposite sides
of the device when a voltage potential is applied.

[0004] There are several different types of EPDs, such as the conventional
type EPD, the microcapsule-based EPD or the EPD with electrophoretic
cells that are formed from parallel line reservoirs. EPDs comprising
closed cells formed from microcups filled with an electrophoretic fluid
and sealed with a polymeric sealing layer are disclosed in U.S. Pat. No.
6,930,818, entitled "Electrophoretic Display and Novel Process for Its
Manufacture", issued on Aug. 16, 2005 to the assignee hereof, the entire
contents of which is hereby incorporated herein by reference for all
purposes as if fully set forth herein.

[0005] Electrophoretic type displays are often used as an output display
device for showing a sequence of different or repeating images formed
from pixels of different colors. Because the history of voltage potential
levels applied to generate the images is different for each pixel, the
voltage potential stress on each pixel of the display is typically
different. These differences from pixel to pixel, in general, lead to
long term issues with image uniformity. Although attempts have been made
previously to alleviate such problems with waveforms that have no DC bias
or by use of clearing images to reduce non-uniformity, neither of these
approaches provides a practical solution to such problems for the long
term.

SUMMARY OF THE DISCLOSURE

[0006] This disclosure is directed toward driving methods which are
particularly suitable for electrophoretic (bi-stable) displays and which
provide the fastest and most pleasing appearance to a desired image while
maintaining optimal image quality over the life of an electrophoretic
display device.

[0007] A first embodiment is directed toward a driving method for a
multi-pixel electrophoretic display comprising a plurality of individual
pixels, which method comprises applying voltage potentials across a
display medium wherein the net magnitude of the voltage potentials
applied, integrated over a period of time, are substantially equal for
all pixels. The display medium for an electrophoretic display may be an
electrophoretic fluid.

[0008] A second embodiment is directed toward a driving method for a
multi-pixel electrophoretic display comprising a plurality of individual
pixels, which method comprises applying driving pulses to a given pixel
wherein the total number of resets to a first color state and the total
number of resets to a second color state are substantially equal, for the
given pixel over a period of time. If there are more than two color
states, substantially equal numbers of resets to each color state may be
used, for a given pixel.

[0009] A third embodiment is directed toward a driving method for a
multi-pixel electrophoretic display comprising a plurality of individual
pixels, which method comprises applying driving pulses to the pixels
wherein the sums of resets to all states are substantially equal for all
pixels. In a more general case having more than two color states, the
total numbers of resets to all color states are substantially equal for
all pixels.

[0010] A fourth embodiment is directed toward a driving method for a
electrophoretic display comprising a plurality of individual pixels,
which method comprises applying driving pulses to the pixels wherein the
pixels are reset to a given color state after a certain number of the
driving pulses.

[0011] A fifth embodiment is directed toward a driving method for a
multi-pixel electrophoretic display comprising a plurality of individual
pixels, which method comprises applying driving pulses to the pixels
wherein the pixels have the substantially equal numbers of resets to each
color state. As in the other embodiments listed above, this method can be
generalized to more than two color states.

[0012] A sixth embodiment is directed toward a driving method for a
multi-pixel electrophoretic display device, in which a corrective
waveform is applied to ensure global DC balance (i.e., the average
voltage potential applied across the display is substantially zero when
integrated over a period of time) or to correct any of the imbalance in
the first, second, third, fourth or fifth embodiment of the disclosure as
described above. The corrective waveform is applied without affecting or
interfering with the driving of individual pixels to intended images and
may be applied at a time when the electrophoretic display would not
normally be in the process of being viewed by a viewer.

[0013] The driving methods of the present disclosure can be applied to
drive electrophoretic displays including, but not limited to, one time
applications or multiple display images (i.e., burst mode display
application). They also could be used with many other display types which
potentially suffer from the same lifetime issues.

[0014] In a further embodiment, a bi-stable driving circuit is provided
which is suitable for implementing the various driving methods disclosed
herein.

[0015] The whole content of each of the other documents referred to in
this application is also hereby incorporated by reference into this
application in its entirety for all purposes as if fully set forth
herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a cross-section view of a typical electrophoretic display
device.

[0017] FIG. 2a and FIG. 2b illustrate a one time display driving
implementation.

[0018] FIG. 3 illustrates an alternative driving implementation for a one
time display.

[0019]FIG. 4 is a diagram which shows how multiple messages may be
displayed in succession.

[0025] FIG. 1 illustrates a typical array of electrophoretic display cells
10a, 10b and 10c in a multi-pixel display 100 which may be driven by the
various driving implementations presented herein, In FIG. 1, the
electrophoretic display cells 10a, 10b, 10c, on the front viewing side,
are provided with a common electrode 11 (which is usually transparent).
On the opposing side (i.e., the rear side) of the electrophoretic display
cells 10a, 10b and 10c, a substrate (12) includes discrete electrodes
12a, 12b and 12c, respectively, Each of the discrete electrodes 12a, 12b
and 12c defines an individual pixel of the multi-pixel electrophoretic
display 100, in FIG. 1. However, in practice, a plurality of display
cells (as a pixel) may be associated with one discrete pixel electrode.

[0026] An electrophoretic fluid 13 is filled in each of the
electrophoretic display cells 10a, 10b, 10c. The discrete electrodes
12a, 12b, 12c may be segmented in nature rather than pixellated, defining
regions of an image to be displayed rather than individual pixels.
Therefore, while the term "pixel" or "pixels" is frequently used in this
disclosure to illustrate driving implementations, the driving
implementations are also applicable to segmented displays.

[0027] Each of the electrophoretic display cells 10a, 10b, 10c is
surrounded by display cell walls 14. For ease of illustration of the
methods described below, the electrophoretic fluid 13 is assumed to
comprise white charged pigment particles 15 dispersed in a dark color
electrophoretic fluid 13.

[0028] The white charged particles 15 may be positively charged so that
they will be drawn to a discrete pixel electrode 12a, 12b, 12c or the
common electrode 11, whichever is at an opposite voltage potential from
that of white charged particles 15. If the same polarity is applied to
the discrete pixel electrode and the common electrode in a display cell,
the positively charged pigment particles will then be drawn o the
electrode which has a lower voltage potential.

[0029] In another embodiment, the white charged pigment particles 15 may
also be negatively charged.

[0030] Also, as would be apparent to a person having ordinary skill in the
art, the white charged particles 15 could be replaced with charged
particles which are dark in color and an electrophoretic fluid 13 that is
light in color so long as sufficient contrast is provided to be visually
discernable.

[0031] In a first embodiment, the electrophoretic display 100 could also
be made with a transparent or lightly colored electrophoretic fluid 13
and charged particles 15 having two different colors carrying opposite
particle charges, and/or having differing electro-kinetic properties.

[0032] The electrophoretic display cells 10a, 10b, 10c may be of a
conventional walled or partition type, a microencapsulted type or a
microcup type. In the microcup type, the electrophoretic display cells
10a, 10b, 10c may be sealed with a top sealing layer. There may also be
an adhesive layer between the electrophoretic display cells 10a, 10b, 10c
and the common electrode 11.

[0033] In one embodiment, a driving implementation for an electrophoretic
display 100 comprising pixels is disclosed. In this embodiment, varying
voltage potentials are applied across the electrophoretic fluid 13 such
that the net vector magnitudes of the voltage potentials applied to the
individual pixels 12a, 12b, 12c, when integrated over a period of time,
are substantially equal for all pixels 12a, 12b, 12c of the
electrophoretic display 100. In this embodiment, variations in the net
vector magnitudes of the voltage potentials applied to the individual
pixels 12a, 12b, 12c when integrated over a period of time should be
maintained within a tolerance of about 20%. However, tighter tolerances
in the net vector magnitudes of the applied voltage potentials of less
than about 10% provides improved image quality and possibly longer
electrophoretic display life. Ideally, tolerances in the net vector
magnitudes of the applied voltage potentials in a range of 0-2% provides
the greatest improvement in displayed image quality but may require more
costly electronics to maintain tolerances in this range.

[0034] In a second embodiment, a driving implementation for an
electrophoretic display 100 comprising pixels 12a, 12b, 12c utilizes
driving pulses applied to a given pixel 12a, 12b, or 12c in order to
maintain a cumulative number of "resets" between a first and second color
state for the given pixel to be maintained substantially equal over a
period of time.

[0035] The term "reset" is defined as applying a driving voltage pulse to
the given pixel to cause the given pixel to change from an original color
state to a different color state or from an original color state to a
different shade of the original color state. The reset may occur as part
of the driving voltage pulse method to cause images to change in the
course of normal pixel operation, for the reduction of flicker effects or
may be used to correct for "history effects" provided by the passive and
persistent display nature of electrophoretic type displays. For
correction of "history effects," the reset may occur when the
electrophoretic display 100 is not in active use or idle. The driving
voltage potential pulse is applied across the electrophoretic fluid 13.

[0036] Since there are many different ways in which a reset can be
accomplished, and since the different types of resets have different
impacts on the uniformity and lifetime of a multi-pixel electrophoretic
display 100, only some of the reset scenarios may be implemented in the
methods described herein; depending on the time required for
implementation and on the cost of implementation. The following table
illustrates different reset scenarios for the term "reset":

[0037] The term "intermediate" color state, in the context of the present
disclosure, is a mid-tone color between a first color state and a second
color state or a composite color of the first and second color states.
For ease of illustration and understanding, it is assumed in the above
Table 1 that the first and second color states are white and dark.
However, it is understood that in a two color display system, the two
colors may be any two colors so long as they provide sufficient contrast
to be differentiated by visual observation.

[0038] In the driving implementation discussed above, a pixel 12a, 12b, or
12c may have N1 number of resets to the white state and N2
number of resets to the dark state where the number N1 and N2
are substantially equal.

[0039] However, depending on the reset scenario selected, the resets may
be counted differently. For example, if Reset Scenario I is selected,
only the "dark to white" and "white to dark" are counted and, in other
words, a pixel has N1 switches from "dark to white" and N2
switches from "white to dark".

[0040] Alternately, if Reset Scenario IV is selected, the reset to white
will include not only "dark to white" but also "white to white" and the
reset to dark will include not only "white to dark" but also "dark to
dark" and, in this case, the total number of resets from "dark to white"
and "white to white" would be N' and the total number of resets from
"white to dark" and "dark to dark" would be N2. As is apparent, the
ten "reset" may be any one of the possible reset scenarios as described
in Table 1, which are applicable to all driving implementations described
in the present disclosure.

[0041] A third embodiment is directed toward a driving implementation for
an electrophoretic display 100 comprising pixels 12a, 12b, 12c. In this
embodiment, driving pulses are applied to the pixels 12a, 12b, 12c where
the sums of reset to all states are substantially equal, for all pixels.
For example, in this driving implementation, a given pixel may have
N3 number of total resets to a first color state and a second color
state, and where the remaining pixels also have a number of total resets
to the two color states which number is substantially equal to N3.
Furthermore, in this embodiment, the numbers of resets to a particular
color state may be the same or different among various pixels, although
the cumulative number of color resets is substantially the same. For
example, a first pixel may be driven to the first color state 60 times
and to the second color state 40 times while a second pixel may be driven
to the first color state 70 times and to the second color state 30 times.
Both the first and second pixels are driven to alternate color states
100 times but not necessarily to the first and second color states
equally.

[0042] In a fourth embodiment, a driving implementation for a
electrophoretic display 100 comprising pixels 12a, 12b, 12c, is provided
where the pixels are reset to a pre-determined color state after a
certain number of driving pulses have been applied to the pixels without
regard to any particular pixel. For example, a reset to each pixel's
original color is provided after 10,000 driving pulses have occurred.
Alternately, rather than counting the number of driving pulses, all
pixels may be driven to a pre-determined color state based on a
pre-determined amount of operating time. In this alternate embodiment,
all of the pixels may not have been applied substantially equal numbers
of driving pulses before they are driven to the pre-determined reset
state.

[0043] In another alternate embodiment, each pixel is reset to a
pre-determined color state when a pre-determined number of driving pulses
have been received. However, since the operation of individual pixels
varies, not all pixels will be driven to the reset color state at about
the same point in time.

[0044] In a fifth embodiment, a driving implementation for a
electrophoretic display 100 comprising pixels 12a, 12b, 12c is provided
where the pixels are voltage potential driven to have substantially equal
numbers of resets to each color state. For example, a given pixel may
have N4 number of resets to a first color state and N5 number
of resets to a second color state; likewise, in this embodiment, the
remaining pixels also have a number of resets substantially equal to the
first and second color states of N4 and N5, respectively. As is
apparent in this fifth embodiment, the pixels are voltage pulse driven
such that the number of resets to the first and second color states are
substantially equal.

[0045] For example, if Reset Scenario V is selected, all pixels are
voltage pulse driven to have N4 resets to the white state (including
"dark to white" and "intermediate to white") and N5 resets to the
dark state (including "white to dark" and "intermediate to dark"). In a
further example, if Reset Scenario VII is selected, all pixels are
voltage pulse driven to have N4 resets to the white state (including
"dark to white", "intermediate to white" and "white to white") and
N5 resets to the dark state (including "white to dark",
"intermediate to dark" and "dark to dark"). In all of these examples,
N4 is substantially equal to N5.

[0046] In this and other embodiments, variation in the number of resets is
intended to be maintained within a tolerance of about 20%. However,
tighter tolerances in the number of resets of less than about 10%
provides improved image quality and possibly longer electrophoretic
display life. Ideally, tolerances in the number of resets in a range of
0-2% provides the greatest improvement in displayed image quality but as
discussed previously may be more costly to implement.

[0047] In a sixth embodiment, a corrective waveform is applied to the
common electrode 11 and the individual pixel 12a, 12b, 12c electrodes to
ensure global DC balance of the electrophoretic fluid 13 contained in
each electrophoretic cell 10a, 10b, 10c. The corrective waveform attempts
to normalize the voltage potentials applied to the electrophoretic fluid
13 so that substantially a net zero volts exist when integrated over a
period of time. The global DC balance is considered to be sufficiently
obtained if an imbalance of less than 90 voltsec (i.e., 0 to about 90
voltsec) is accumulated over a period of at least about 60 seconds.
Improved results are realized if the imbalance of less than 90 voltsec is
achieved over a range of about 60 minutes to about 60 hours. The
application of the corrective waveform assists in maintaining uniformity
of the electrophoretic fluid 13 among all of the electrophoretic cells
10a, 10b, 10c of the multi-pixel electrophoretic display 100. The
corrective waveform may also be applied in addition to any of the pixel
reset scenarios discussed above in the first, second, third, fourth or
fifth embodiment. The corrective waveform is typically applied at a later
time so that it does not interfere with the driving of pixels to intended
images. The global DC balance and other types of balance as described in
the present disclosure are important for maintaining maximum long term
contrast and freedom from residual images.

[0048] In this embodiment of the disclosure, programmable circuits are
used to correct for the DC imbalance at periodic intervals utilizing a
corrective equalizing waveform. For example, a microcontroller 800 (FIG.
8) may be used to keep track of the level of DC imbalance, and correct
for imbalances on a regular basis. The microcontroller 800 may comprise a
memory element 802 which records the cumulative number of voltage pulses
applied to a given pixel, or a number of resets to a given color state
for each pixel, over a period of time. At some periodic interval (i.e.,
once per predetermined time period, or some time after a sequence of
driving voltage pulse waveforms), a separate corrective waveform may also
be applied which substantially compensates for DC imbalances recorded in
the memory 802. Amore detailed discussion of the microcontroller 800 and
associated circuitry is provided in FIG. 8 below.

[0049] The corrective waveform may be accomplished either at a separate
time when the electrophoretic display 100 would be expected to be idle or
when it would otherwise not interfere with normal driving of intended
pixels (i.e., during normal display), or as an extension of another
predetermined waveform so as to not be visually discernable. For example,
a corrective waveform is provided at a duration or rate not discernable
to an Observer. Several embodiments of this corrective driving
implementation can be envisioned, depending on the intended applications.
A few of these are described below. However, a person having ordinary
skill in the art will appreciate that many variations of the methods
disclosed below may be provided.

[0050] In a first embodiment, a corrective waveform is used and imbalances
in pixels 12a, 12b, 12c may be corrected at a time when an
electrophoretic display 100 is not in operation, for example, in the
middle of the night or at a predetermined time when the electrophoretic
display 100 is not expected to be in use. Although many applications are
perceived for this method of achieving the balance, a smartcard having an
integrated electrophoretic display 100 or other similar security token
devices are examples which may benefit from a corrective waveform. For
example, when a smartcard is used, a user wants to review the displayed
information as quickly and easily as possible, but following use, the
smartcard is then typically disposed in the user's wallet for the
majority of time, so that a corrective waveform applied at a later time
will rarely be observed by the user.

[0051] In a second embodiment, no corrective waveform is required.
Instead, a longer driving voltage potential pulse is applied. This
approach is particularly useful if the longer driving voltage potential
pulse is at the end of a normal driving sequence so that there would be
no visual impact on the image displayed. The additional amount of time
required for the driving pulse is determined by a microcontroller 800 and
should be sufficiently long in order to compensate for the imbalance
which have been stored in the memory 802 of the microcontroller 800 based
on the driving history or changes in color state of the pixels 12a, 12b,
12c (FIG. 1)

[0052] An imbalance of too many white pixels may be corrected by applying
a longer driving pulse when the white pixels are driven to the dark
state, especially if the dark state occurs at the end of a normal driving
sequence. Such a corrective waveform extension can be used to correct for
DC imbalance or net vector magnitudes of applied voltage potentials to
the pixels 12a, 12b, 12c as discussed above. In embodiments of the
disclosure involving equalization of the number of resets, the extended
corrective waveform comprises a number of resets used to achieve the
correction. This embodiment of the disclosure is demonstrated in Example
5 below.

[0053] In a third embodiment of this corrective driving implementation,
the DC imbalance may also be corrected with a color flash (i.e., driving
all pixels to a predetermined color state, sometimes referred to as a
"white flash,") at the beginning of the next sequence of normal display
waveforms. For normalizing the global DC balance, this will allow for a
zero time average DC bias and help to display cleaner images. However,
this driving implementation may provide an undesirable initial display
flash at the time of initiation of the next sequence of waveforms.

[0054] The driving implementations of the present disclosure are
applicable to a variety of electrophoretic displays. In an
electrophoretic display 100 with a traditional up-down switching mode,
the charged pigment particles 15 move in a vertical direction between the
electrodes 11 and 12a, 12b, 12c as shown in FIG. 1, depending on the
voltage potentials applied to the electrode layers 11 and 12a, 12b, 12c .
If the electrophoretic display fluid 13 comprises charged white particles
15 dispersed in a dark color fluid, the images displayed by this
electrophoretic display 100 would be in white/dark colors.

[0055] The driving implementations of the present disclosure may also be
applied to an electrophoretic display with an in-plane switching mode,
Examples of in-plane switching electrophoretic display are described in
E. Kishi, et al., "5.1: Development of In-plane EPD", Canon Research
Center, SID 00 Digest, pages 24-27 (2000); Sally A. Swanson, et al.
(2000); "5.2: High Performance EPDs", IBM Almaden Research Center, SID 00
Digest, pages 29-31 (2000); and U.S. Pat. No. 6,885,495, entitled
"Electrophoretic Display with In-plane Switching", issued Apr. 26, 2005,
to the assignee hereof, the entire contents of all the above documents
are incorporated by reference herein in their entirety as if fully set
forth herein. A typical in-plane switching electrophoretic display may
also exhibit two contrasting colors.

[0056] Furthermore, the driving implementations described herein may also
be adapted to a electrophoretic display which is capable of displaying
more than two color states, such as a dual mode electrophoretic display
as described in U.S. Pat. No. 7,046,228, entitled "Electrophoretic
Display with Dual Mode Switching," issued on May 6, 2006 to the assignee
hereof, the content of which is herein incorporated by reference in its
entirety for all purposes as if fully set forth herein.

EXAMPLES

[0057] For ease of illustration and understanding of the various
corrective waveforms of the present disclosure, a set of drawings is
provided in FIG. 2 to FIG. 7. With respect to FIG. 2 to FIG. 7, the
electrophoretic display 100 (FIG. 1) is assumed to be comprise white
charged pigment particles 15 dispersed in a dark electrophoretic color
fluid 13 and the particles 15 are positively charged so that they will be
drawn to a discrete pixel electrode 12a, 12b, 12c or the common electrode
11, whichever has an opposite polarity or at a lower voltage potential.

Example 1

One Time Display Implementation

[0058] In this example, some of the images would be displayed on the
electrophoretic display 100 only once. For one time display
implementations, the displayed image on the electrophoretic display 100
is to be turned off or cleared after a pre-determined display period, for
example, a one time password used in a smartcard application. After the
onetime password is generated and displayed, the password image should be
cleared for security reasons. In this implementation, the electrophoretic
display 100 will be driven to the dark state and then wait for the next
driving sequence.

[0059]FIG. 2 illustrates one of the onetime display driving embodiments,
In this embodiment, the initial color state or the "off" state of the
electrophoretic display 100 is represented by the dark color state of the
electrophoretic fluid 13 (display medium.) As depicted, the driving
implementation has two phases, a driving phase and a clearing phase. The
driving phase is shown in FIG. 2a. The clearing phase, as shown in FIG.
2b, has two frames 201 and 202. The top waveform in FIG. 2a shows that no
voltage potential is applied to the common electrode in the driving
phase. Waveform I in FIG. 2a shows a voltage potential of +V is applied
to drive the white pixels from the dark state (i.e., "off state") to the
white (visible) state. Waveform II shows that no voltage potential is
applied so that the dark pixels remain in the dark state during the
driving phase.

[0060] In the clearing phase as shown in FIG. 2b, no voltage potential is
applied in frame 201 and a voltage potential of +V is applied in frame
202, to the common electrode 11 (FIG. 1) For the white pixels to be
cleared, initially no voltage potential is applied across the display
medium 13 in frame 201 and the white pixels remain white in frame 201
followed by a voltage potential of -V (shown as a net "0" V value) being
applied across the display medium 13 in frame 202 which causes the white
pixels to revert to the dark state (the "off" state) in frame 202. In
this approach the common is +V and the pixel is 0, and therefore the net
voltage potential is -V. For the dark pixels to be cleared (i.e., to
remain dark in the dark state), a voltage potential of +V is applied
across the display medium 13 in frame 201 which drives the dark pixels to
the white state in frame 201 and a voltage potential of -V (shown as a
net "0" V value) is applied across the display medium 13 in frame 202
which drives the dark pixels back to the dark "off" state in frame 202.
Therefore at the end of the clearing phase, both the white and dark
pixels are returned to the original dark "off" state. In the driving
implementation of FIG. 2, when the duration of the driving phase of FIG.
2a is substantially equal to that of frame 202 shown in FIG. 2b and the
durations of the frames 201 and 202 are also substantially equal, a
global DC balance can be achieved. The driving implementation of FIG. 2
also represents the first embodiment of the disclosure, that is, the net
vector magnitudes of the voltage potentials applied, integrated over a
period of time, are substantially equal for all pixels (i.e., white and
dark), provided that when the duration of the driving phase is
substantially equal to that of frame 202 and the durations of the frames
201 and 202 are also substantially equal.

[0061] The driving implementation of FIG. 2 also represents the second
embodiment of the disclosure, that is, the number of resets to the white
state (D to W) is equal to the number of resets to the dark state (W to
D), for each pixel. The driving implementation of FIG. 2 further
represents the third embodiment of the disclosure, that is, the total
number of resets to the dark state and to the total number of resets to
white state are the same for both white and dark pixels (i.e., 2). The
driving implementation of FIG. 2 further represents the fourth embodiment
of the disclosure, that is, all pixels are reset to the dark state after
a series of driving pulses.

[0062] The driving implementation of FIG. 2 further represents the fifth
embodiment of the disclosure as all pixels have the same number of resets
to the white state and the same number of resets to the dark state.

Example 2

Alternative One Time Display Implementation

[0063] Experience has shown that if an electrophoretic display remains
inactive for an extended period of time, the performance of transitioning
from the dark state to the white state or vice versa may become degraded,
and the dark state may have assumed a less than optimal charge value.
FIG. 3 illustrates an alternative driving phase to that in FIG. 2 to
address this issue. As shown in the FIG. 3, the driving phase in this
alternative implementation has two driving frames, 301 and 302. For the
common electrode 11 (FIG. 1) in this driving implementation, no voltage
potential is applied in driving frame 301 and a voltage potential of +V
is applied in driving frame 302. Waveform I drives pixels from the dark
"off" state to the white state by applying across the display medium 13 a
voltage potential of +V frame 301 and no voltage potential in frame 302
and as a result, the pixels switch to the white state in frame 301 and
remain in the white state in frame 302. Waveform II, on the other hand,
keeps pixels in the dark state by applying across the display medium no
voltage potential in frame 301 and a voltage potential of -V (shown as a
net "0" V value) in frame 302 and in this case, the dark pixels remain
dark in driving frame 301 and further driven to the dark state in frame
302. The addition of the driving frame 302 has the effect of improved
contrast ratio, especially if the electrophoretic display has undergone a
prolonged period of inactivity. The clearing phase of this implementation
is the same as that of FIG. 2b.

[0064] The duration of driving frame 301 does not have to be equal to the
duration of driving frame 302. However, in order to maintain the global
DC balance discussed above, the duration of frame 301 is generally
maintained substantially equal in duration to that of the frame 202.
Accordingly, the duration sum of driving frame 302 and frame 202 (FIG. 2)
are substantially equal to the duration of frame 201.

Example 3

Multiple Message Display Implementation

[0065] An electrophoretic display may display multiple images
sequentially. The multiple messages may be shown in sequence within a
short period of time (e.g., 1-2 minutes) and the final message may remain
for a longer period of time unless cleared or corrected. The multiple
messages may be displayed one after another or the multiple messages may
be a repeat of two or more messages, switching back and forth as driven
by a microcontroller 800 (FIG. 8)

[0066]FIG. 4 depicts an example as to how multiple messages may be
displayed in succession. In the sequence as shown, the "idle" time
between messages is optional. The final message in the sequence may
remain for a period of time, if needed. A corrective waveform may be
applied between messages (not shown) or after the second message has been
displayed to drive the white pixels to the dark state and provide DC
balancing as briefly discussed above and discussed in more detail with
respect to FIG. 5 below. To illustrate a clear example, FIG. 4 shows two
messages followed by a correction, but other embodiments may use three or
more messages.

[0067] FIG. 5 depicts one of the driving implementations for multiple
messages. For exemplary purposes, FIG. 5a, FIG. 5b, and FIG. 5c provide a
string of three consecutive messages, First Message, Second Message and
Third Message. Each of the messages is provided with a clearing phase and
a driving phase. For all three messages in this implementation, the
common electrode 11 (FIG. 1) is always applied a voltage potential of +V
in the clearing phase and no voltage potential is applied in the driving
phase.

[0068] In FIG. 5a (First Message), for Waveform I representing white
pixels to remain in the white state, a voltage potential of -V (shown as
a net "0" V value) is applied across the display medium 13 in the
clearing phase and a voltage potential of +V is applied across the
display medium 13 in the driving phase, and in this case, the white
pixels are driven to the dark state in the clearing phase and then back
to the white state in the driving phase. For Waveform II representing
white pixels to driven to the dark state, a voltage potential of -V
(shown as a net "0" V value) is applied across the display medium 13 in
the clearing phase and no voltage potential is applied across the display
medium 13 in the driving phase, and as a result, the white pixels are
driven to the dark state in the clearing phase and remain in the dark
state in the driving phase. For Waveform III representing dark pixels to
be driven to the white state, no voltage potential is applied across the
display medium 113 in the clearing phase and a voltage potential of +V is
applied across the display medium 13 in the driving phase, and in this
case, the dark pixels remain in the dark state in the clearing phase and
are driven to the white state in the driving phase. For Waveform IV
representing dark pixels to remain in the dark state, a voltage potential
of -V (shown as a net "0" V value) is applied across the display medium
13 in the clearing phase and no voltage potential is applied across the
display medium in the driving phase, and as a result, the dark pixels
remain in the dark state in both the clearing and driving phases. The
Third Message (FIG. 5c) has the same driving waveforms as the First
Message (FIG. 5a). However, the Second Message, between the First and
Third Messages has different waveforms from I and IV.

[0069] In FIG. 5b (Second Message), Waveforms II and III are the same as
those of FIG. 5a and FIG. 5c. However, for Waveform I representing white
pixels to remain white, no voltage potential is applied across the
display medium 13 in either the clearing or driving phases, and in this
case, the white pixels remain white in the clearing and driving phases.
For Waveform IV representing dark pixels to remain in the dark state, no
voltage potential is applied across the display medium 13 in both the
clearing and driving phases, and as a result, the dark pixels remain in
the dark state in the clearing and driving phases.

[0070] The driving implementation as depicted in FIG. 5 has certain
features. For example, no pixels need to be driven if there is no color
state change in the Second Message (see Waveforms I and IV of FIG. 5b).
If there is a required change in the color state in pixels caused by the
Second Message, the pixels are driven to the desired color state
accordingly. In the First and Third Messages, a white pixel remaining in
the white state is driven to the dark state first and back to the white
state and a dark pixel remaining in the dark state is re-driven to the
dark state first, to ensure refreshing of the dark pixels. Depending on
the implementation, an idle time may be provided between each of the
messages. The idle time, as stated above, is optional.

[0071] The driving implementation for multiple messages as described in
this example has many advantages. For example, only pixels having color
state change in consecutive messages are driven. Therefore, the image
change may occur at a high speed. In addition, the driving implementation
also provides refreshing of pixels to maintain good bistability. A
corrective waveform may be added at the end of the driving sequence to
correct any DC imbalances (see Examples 4 and 5 below) occurring from
non-uniform pixel operation.

Example 4

Offline Corrective of Global DC Balance

[0072] In this example, the Waveforms I-IV described above for FIG. 5a,
FIG. 5b, and FIG. 5c are used to illustrate the use of a post corrective
waveform. The driving implementation of Example 3 above provides a very
clean image switching sequence for displaying multiple messages; however,
this implementation could generate a DC imbalance which if left
uncompensated, could cause image degradation in some circumstances.

[0073] Table 2 shows various combinations of driving scenarios for a
string of three messages. According to Table 2, the waveforms of Example
3 (see FIG. 5) may give a maximum imbalance, at the end of the entire
sequence, of 1(-V), 0 or 1(+V), assuming that all the driving and
clearing waveform elements have the same duration (t0).

[0074] FIG. 6 shows the waveforms for correcting the DC imbalance when the
corrective waveforms are initiated at some time after the end of the last
message set (Third Message), for example, after 30 seconds. If there is
no DC imbalance in the driving sequence for a given pixel, such as that
shown in the rows in Table 2 with zero DC offset, the corrective Waveform
6a (FIG. 6) may be applied which does not impact any currently displayed
images. If the desired end state is dark and there is an imbalance of one
dark pixel 1(-V), the corrective Waveform 6b may be applied. If the
desired end state is dark and there is an imbalance of one white pixel
1(+V), Waveform 6c may be applied. If the desired end state is white and
there is an imbalance of one white pixel 1(+V), the corrective Waveform
6d may be applied. If the desired end state is white and there is an
imbalance of one dark pixel 1(-V), then Waveform 6e may be applied. The
combined set of waveforms shown in FIG. 5a, FIG. 5b, FIG. 5c and FIG. 6
will correct the DC imbalance.

[0075] When any of the corrective waveforms is applied, if for any reason,
there is another message demand before, for example, the 30 second
interval, that message demand would override the corrective waveform and
display the additional message, and after that second message is complete
and another 30 seconds has expired, then one of appropriate corrective
waveforms is applied a sufficient number of times to correct for the net
imbalance achieved since the last correction. If the additional message
causes additional imbalances, for example, of 1(-V), the Waveform 6b or
6e may then need to be applied twice to correct the imbalance of 2(-V).
The example only demonstrates a few possible corrective waveforms, which
can be modified or extended in a wide number of corrective waveforms to
compensate for different levels of DC imbalance. In a similar manner, any
of the imbalances in the first through fifth embodiments of this
disclosure may also be corrected.

Example 5

[0076] In another corrective waveform technique, rather than adding a
separate corrective waveform, the existing waveforms are extended to
correct a DC imbalance which can be achieved in a way not visually
discernable. For example, FIG. 5d shows an extended version of the Third
Message of FIG. 5c. In FIG. 5d, a set of waveforms "Extension DD" is
added between the original clearing and driving phases and another set of
waveforms "Extension WW" is added after the driving phase. In the
extended phases, each waveform is presented with two options, shown as
the solid and dotted lines. The dotted lines indicate that the voltage
potentials for the dark or white states have been extended in time to
correct an imbalance from previous messages. The solid lines indicate
that a waveform in which no voltage potential difference is applied
across the display medium, so that no change in the image state occurs
and no visible impact on the images displayed is observed, except that
the time of the waveforms for the Third Message is lengthened to allow
dotted frames DD or WW. As is apparent from this waveform, not every
pixel can be corrected in this way. For example, in Waveforms II and IV,
the pixels in the dark state cannot be corrected with extended Waveforms
WW; and as a result, they cannot be balanced until subsequent waveforms
are applied in which a corrective opportunity occurs. The microcontroller
800 (FIG. 8) simply keeps track of which pixels need to be corrected and
adds the extra length of waveforms at an opportune time.

[0078] The present techniques may be applied to a wide variety of the
electronic devices. The smartcard is one of many examples. The smartcard
can be used for any application requiring information to be displayed
including, but not limited to, a stored value from an internal memory of
the device, a generated password from the internal electronics of the
device and a transferred value from an external device to the smartcard.

[0079] Referring to FIG. 7, a process flow chart is shown for implementing
one or more of the disclosed embodiments. The process is initiated at
block 700 and continues to block 705. At block 705, a microcontroller 800
(FIG. 8) waits for a message to be received from the device circuit 815
(FIG. 8). When a message is received at block 710 by the microcontroller
800 from the device circuit 815, the message is output to the
electrophoretic display at block 715 by the microcontroller 800. At block
720, the microcontroller 800 records certain parameters associated with
the driving pulses applied to the pixels needed to display the message
output at block 715.

[0080] At block 725, the microcontroller 800 determines whether another
message is to be output to the electrophoretic display 100 (FIG. 1) If
another message is to be output 725, the microcontroller 800 outputs the
message to the electrophoretic display 100 as before at block 715 and
likewise records the certain parameters in memory 802 associated with the
driving pulses applied to the pixels needed to display the message of
block 715 at block 720.

[0081] At block 725, if another message is not pending for output, the
microcontroller 800 proceeds to block 730 to determine whether a clear
display timer has elapsed. If microcontroller 800 determines that the
clear display timer has not elapsed, the microcontroller 800 waits for
another message to arrive as previously described for blocks 715, 720 and
725. If the microcontroller 800 determines at block 730 that the clear
display timer has elapsed, the microcontroller 800 sends the proper
driving pulses to clear electrophoretic display 100 at block 735. In one
embodiment, the clearing of electrophoretic display 100 at block 735 also
causes the microcontroller 800 at block 740 to reset the clear display
timer to restart timing for clearing the electrophoretic display 100.

[0082] In one embodiment, the microcontroller 800 determines if a display
correction is required at block 745. The display correction at block 745
may be provided to substantially equalize the number of times a driving
pulse is applied to individual pixels, the number of resets to a
particular color state for individual pixels, the number of resets to two
or more color states for the individual pixels and/or correction of a
relative DC imbalance among the individual pixels as described above. At
block 745, if the microcontroller 800 determines that display correction
is not required, the microcontroller 800 returns to block 705 to wait for
a message 820 from the device circuit 815 as previously described.

[0083] At block 745, if the microcontroller 800 determines that display
correction is required, the microcontroller 800 proceeds to block 750
which applies one or more of the above described display corrections to
the multi-pixel electrophoretic display 100 such as pixel drive pulse
balance 755 and/or DC balance 760.

[0084] In one embodiment, at block 750, once the display correction has
been applied and completed, the microcontroller 800 returns to block 705
to wait for a message 820 from the device circuit 815 as previously
described.

[0085] Referring to FIG. 8, an exemplary block diagram of a
microcontroller circuit suitable for implementing the various embodiments
described is shown. In one embodiment, a microcontroller 800 includes a
memory 802 and an internal clock 804. The microcontroller 800 may be of
any common programmable type such as an ASIC, FPGA, CPLD, LSIC,
microprocessor, programmable logic gate circuit or similar intelligent
devices. The microcontroller 800 is provided with a DC power source 810
typically from a battery. In one embodiment, the microcontroller 800 is
operatively coupled to a bi-stable driver controller 805.

[0086] The bi-stable driver controller 805 converts signals received from
the microcontroller 800 into voltage driving pulses which are supplied to
the bi-stable display 100 by connections 805a, 805b. In one embodiment,
the bi-stable controller provides 50 millisecond (ms) to 500 ms
electrical driving pulses to the bi-stable display 100. In one
embodiment, the multi-pulse voltage driving frames of 200 ms to 1500 ms
are provided by the bi-stable driver controller 805 to the bi-stable
display 100. In one embodiment, the microcontroller 800 and bi-stable
driver controller 805 are integrated into a single form factor. For
example, a field programmable gate array (FPGA) coupled to the bi-stable
display 100 using bipolar op-amps.

[0087] In one embodiment, the bi-stable controller 805 typically includes
a DC-DC converter 807 which is used to increase the voltage supplied
from the DC power source 810 to about 30-40 VDC. The messages 820
received from the device circuit 815 cause microcontroller 800 to signal
the bi-stable controller 805 to output the message 820 to the bi-stable
(electrophoretic) display 100.

[0088] In one embodiment, the microcontroller 800 is provided with logical
instructions to perform the display corrective implementations described
above, including but not limited to, substantially equalizing the number
of times a driving pulse is applied to individual pixels of bi-stable
display 100, the number of resets to a particular color state for
individual pixels of bi-stable display 100, the number of resets to two
or more color states for the individual pixels of bi-stable display 100
and/or correction of a relative DC imbalance among the individual pixels
of bi-stable display 100 as described above.

[0089] Although the foregoing disclosure has been described in some detail
for purposes of clarity of understanding, it will be apparent to a person
having ordinary skill in that art that certain changes and modifications
may be practiced within the scope of the appended claims. It should be
noted that there are many alternative ways of implementing both the
process and apparatus of the improved driving scheme for an
electrophoretic display, and for many other types of displays including,
but not limited to, liquid crystal, rotating ball, dielectrophoretic and
electrowetting types of displays. Accordingly, the present embodiments
are to be considered as exemplary and not restrictive, and the inventive
features are not to be limited to the details given herein, but may be
modified within the scope and equivalents of the appended claims.